Owing to the limited resources of fossil fuels, especially limited reserves of mineral oil as a raw material for the production of fuels for operating internal combustion engines, there are constant efforts to reduce the fuel consumption of internal combustion engines. Here, improved, i.e. more effective, combustion is in the foreground of these efforts. However, certain strategies in respect of the fundamental operation of the internal combustion engine can also be helpful.
One concept for reducing the fuel consumption of a vehicle includes, for example, in switching off the internal combustion engine instead of continuing to operate it at idle when there is no demand for power. In practice, this means that the internal combustion engine is switched off at least when the vehicle is stationary. One application is “stop-and-go” traffic of the kind which occurs, for example, in congestion on freeways and highways. In towns, stop-and-go traffic is no longer the exception but the rule, owing to the uncoordinated traffic light systems which are present. Restricted railroad crossings and the like represent further applications. Within the context of the present disclosure, switching off the internal combustion engine when there is no demand for power, together with restarting, is also referred to as stop-and-go operation of the internal combustion engine.
The problem with concepts which switch off the internal combustion engine in the absence of demand in order to reduce fuel consumption is the need to restart the internal combustion engine.
Restarting presents problems, inter alia, because the crankshaft and the camshaft come to a halt in a random and furthermore unknown position when the internal combustion engine is shut down in an uncontrolled manner. Consequently, the position of the pistons in the at least two cylinders of the internal combustion engine is likewise unknown and is left to chance. However, this information is indispensable for a restart which is uncomplicated and as rapid and therefore as low in fuel consumption as possible.
Markers arranged on the crankshaft and/or on the camshaft can supply signals relating to the crank angle position to sensors connected to the engine controller. However, to generate these signals it is first necessary to set the crankshaft in rotation. If there is a lack of clarity on the crank angle position at the beginning of a restart, a run-in phase to synchronize the engine operating parameters with the crankshaft rotation is necessary.
It is necessary to know the position of the individual pistons of an internal combustion engine, for example, to enable fuel injection to take place selectively, i.e. at defined crank angles. The task of controlling injection is generally performed by an engine controller.
According to the prior art, the signal generated by a crankshaft sensor or camshaft sensor is used by the engine controller to calculate the speed of rotation and angular position of the crankshaft. The engine controller utilizes these data to calculate the fuel injection and quantity of fuel under all operating conditions of the internal combustion engine. A crankshaft sensor is preferably used since the crankshaft revolves at twice the speed of the camshaft and hence supplies a signal with a significantly higher resolution.
The piston position can be determined with significantly greater accuracy with the aid of a crankshaft signal than with the aid of a camshaft signal. The camshaft sensor is preferably used to enable it to be determined whether the cylinder is in the combustion cycle—compression and expansion—or in the charge exchange cycle—exhaust and intake. Since, in the case of a four stroke internal combustion engine, one operating cycle, consisting of compression, expansion, exhaust and intake, comprises 720° of crankshaft angle (KW), it may be necessary to ascertain whether a piston situated at top dead center (TDC) is at “combustion TDC” or at top dead center during charge exchange.
In practice, it is generally the case that the position of just one single cylinder of the internal combustion engines is determined by means of said sensors, thereby establishing the position of the other cylinders.
In order to facilitate restarting, various concepts are proposed in the prior art. German Patent Application DE 42 30 616, for example, proposes to store the angular position of the crankshaft, which is recorded when switching off, and to use it for restarting, ensuring that the appropriate injection timings are immediately available.
Other approaches to a solution give preference to methods for controlled shutdown and starting of the internal combustion engine. Here, controlled shutdown includes deliberately adopting very particular crank angle positions—referred to as preferential positions—when switching off the internal combustion engine. Here, the end position of the crankshaft is no longer left to chance but is deliberately brought about.
However, even if the position of the crankshaft and the position of the pistons at the beginning of a restart are known, the starting operation generally comprises several revolutions of the crankshaft or several operating cycles since the fuel pump may first of all build up in the fuel supply system the pressure required for injection before fuel can actually be introduced into the cylinders by means of fuel injection nozzles. When the internal combustion engine is switched off, it is generally not possible to maintain the pressure in the fuel supply system owing to a lack of leaktightness or to leaks.
Another disadvantage of strategies in which the internal combustion engine is switched off in the absence of demand in order to reduce fuel consumption is that the demands on the starting device are increased by stop-and-go operation. On the one hand, the number of starting operations increases if the internal combustion engine is switched off more frequently, and this requires a correspondingly robust starting device. On the other hand, the starting operation, which can take up to one second, prejudices responsiveness and, due to starting noise, driving comfort.
In order to be able to operate an internal combustion engine in accordance with various regulations, especially in respect of increasing stop-and-go traffic, i.e. to be able to switch it off in the absence of demand, it is thus necessary to simplify restarting, in particular to make it more rapid and more economical with fuel.
Given the background of what has been stated, the inventors herein provide a system to at least partly address the above issues in order to provide an engine restart that may be achieved more quickly and with less use of fuel than in the prior art.
In one embodiment, an internal combustion engine comprises at least two cylinders each including an injection nozzle and a fuel supply system for supplying the cylinders with fuel. The fuel supply system includes a supply line connecting each injection nozzle to a first fuel reservoir storing fuel at a first pressure, the first fuel reservoir filled by a pump provided upstream, a second fuel reservoir storing fuel at a second pressure less than the first pressure and connected to the first fuel reservoir via a connecting line for filling with fuel, and a bypass line connecting the second fuel reservoir to each injection nozzle. The bypass line opens into the fuel supply system downstream of the first fuel reservoir, thereby forming a connection point, and a shutoff element is arranged in the bypass line, opening or shutting off the bypass line.
In contrast to the prior art, the injection of fuel in the context of a restart is possible without delay, i.e. immediately upon initiation of the restart, in the case of the internal combustion engine according to the disclosure. For this purpose, the injection nozzles of the cylinders are supplied, i.e. fed, with fuel from a second fuel reservoir via a bypass line in the starting phase. This secondary fuel system assumes the task of supplying the cylinders or injection nozzles with fuel on an auxiliary basis until the pump has built up a sufficiently high pressure in the actual fuel supply system to be able to supply the cylinders or injection nozzles with fuel once again from the first fuel reservoir in accordance with normal operation of the internal combustion engine. In this case, it is necessary to open the shutoff element arranged in the bypass line.
Whereas the fuel in the first fuel reservoir has to be stored or made available at a sufficiently high pressure p1 to guarantee the fuel supply under all operating conditions in normal operation of the internal combustion engine, in particular also after a cold start, it is sufficient to store the fuel in the second fuel reservoir at a lower pressure p2 in order to supply the already preheated cylinders with fuel for n operating cycles in the context of stop-and-go operation.
Moreover, the lower pressure p2 in the secondary fuel system, that is used on an auxiliary basis, makes it possible to maintain this pressure p2 in the second fuel reservoir by shutting off the bypass line, i.e. by closing the shutoff element arranged in the bypass line, until a restart in the context of stop-and-go operation. Owing to the lower pressure level, there is no reduction in the fuel pressure of the kind that can be observed in the actual fuel supply system when the internal combustion engine is switched off. The second fuel reservoir can be connected to the first fuel reservoir via a connecting line for filling with fuel.
The internal combustion engine according to the disclosure allows a rapid restart using less fuel. Further advantages are obtained in respect of the quantities of pollutants generated during starting and in respect of a starting device used to assist the starting operation, which is superfluous and can be deactivated only a short time after the initiation of the restart, owing to the acceleration of the starting operation. Shortening the starting time improves responsiveness and, in particular, also driving comfort owing to lower noise emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.